Internal combustion engines fueled by gasoline or diesel, either alone or in combination with other compounds (e.g., ethanol), are sources of large amounts of air and other environmental pollutants that impact the health and lifespan of the world human population, as well as the environment as a whole. Thus for example carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxide (SO2), non-methane hydrocarbons (NMHCs) and particulate matter (PM) are produced as a result of the combustion of gasoline or diesel fuel in stationary, on-road, and off-road engines. Despite decades of regulation, these and other pollutants continue to be released into the environment in amounts that exceed regulatory standards, even in countries with strict emission controls. In the United States, for example, even after decades of stringent air pollution regulations, over 150 million people still live in areas where pollution levels exceed national ambient air quality standards (NAAQS). See, e.g., epa.gov/airtrends/sixpoll.html.
In order to reduce the amounts of combustion engine-generated pollutants, increasingly stringent gas- and diesel-fueled engine emissions standards have been enforced in the United States and abroad. Unfortunately, robust technologies for meeting these standards have been difficult to obtain. Particulate matter emission levels in the United States alone, for example, although reduced by 33% and 51% between 1990 and 2007 for large and small particulates (PM10 and PM2.5; see below for definitions), even now still amount to over 1 million tons of particulates released into the environment in the United States per year, with about a third of these particulates resulting from highway and off-road vehicle engine sources, most of which are equipped with emissions control devices. See, e.g., epa.gov/air/airtrends/2008/report/Highlights.pdf, the contents of which are herein incorporated in their entirety by reference.
One technology that offers great promise for reducing combustion-engine emissions, particularly particulate matter emissions, is based on the use of non-thermal plasma (NTP) to increase combustion efficiency and/or to improve the reduction of tailpipe emissions. Specifically, plasma is generally defined as an ionization gas, where atoms, positive and negative ions and electrons are intermingled, but which is, in the aggregate (i.e., at a bulk matter or macroscopic level), electrically neutral. See, e.g., Yamamoto and Okubo, Nonthermal Plasma Technology, in the Handbook of Environmental Engineering, Vol. 5: Advanced Physiochemical Treatment Technologies, Humana Press (synonymously, “Yamamoto and Okubo”), the contents of which are herein incorporated in its entirety by reference. “Thermal plasmas” are plasmas where the temperatures of the constituent atoms, ions and electrons of the plasma are the same (i.e., at thermal equilibrium and “hot”); non-thermal plasmas (NTPs) are typically non-equilibrium plasmas where the electrons are “hot” while the other species in the plasma are thermally “cold.” Electric arcs are an example of thermal plasmas; low temperature devices, such as neon lamps, are an example of non-thermal plasmas.
With regard to combustion efficiency, preliminary studies indicate that NTPs can be used to break up large organic fuel molecules into smaller molecules that are more easily and completely combusted; see, e.g., U.S. Patent Publication Nos. 2004/0185396, 2005/0019714, and 2008/0314734, the contents of which are herein incorporated in their entireties by reference. The result of such increases in combustion efficiency is improved fuel consumption and, indirectly, potentially fewer tailpipe emissions.
Other studies have shown that NTPs can be used to directly reduce tailpipe emissions. For example, in a large number of studies NTPs have been generated by a variety of means in systems that aim to reduce NOx emissions. See, e.g., U.S. Pat. Nos. 6,482,368 and 6,852,200, the contents of which are herein incorporated in their entireties by reference. Other systems use NTP to reduce particulate matter (PM). See, e.g., U.S. Pat. No. 5,263,317 and U.S. Patent Publication No. 2007/0045101, the contents of which are herein incorporated in their entireties by reference.
Despite the attractiveness of NTP-based systems for reducing engine emissions, the use of this technology has been complicated by the effects of the pollutants and breakdown products in exhaust gases on these systems. In this regard, particulate matter is especially problematic, since its accumulation in these systems can cause the physical blockage of narrow gas-flow regions. Such PM can also degrade or destroy the performance of NTP systems by coating the components involved in the generation of NTP. In the case of systems where NTP is generated electrically, PM accumulation can cause power losses via the redirection of current into conductive paths created by the accumulation of this conductive material, thereby reducing the amount of NTP generated, and therefore the amount of particulate matter removed. Another concern is the amount of power consumed to reduce the PM. Existing NTP systems can consume hundreds of watts of power just to reduce PM by 25%. Energy used to power the system reduces the power available for other uses and increases the emissions. NTP systems with a greatly increased reduction per watt of power used are therefore needed to reduce total emissions.
In light of the above, it is clear that there is a need for better NTP-based pollution reduction systems, particularly NTP-based systems in which PM accumulation within such systems is efficiently reduced or prevented.